(a) Field of the Invention
The present invention relates to an organic electroluminescent (EL) device having an excellent luminescence property such as high efficient luminance and a method for fabricating the same.
(b) Description of the Related Art
An organic electroluminescent device (which will hereinafter be called xe2x80x9corganic EL devicexe2x80x9d) is a light-emitting device which makes use of the principle that when an electric field is applied, a fluorescent material emits light in response to the charge recombination of holes injected from an anode and electrons injected from a cathode. After C. W. Tang et al. of Eastman Kodak Company reported a low-voltage-driven organic EL device using a double layered structure (C. W. Tang, S. A. Vanslyke, Applied Physics Letters, 51, 913(1987) and the like), studies on an organic EL device have been briskly carried out. Tang et al. reported an organic EL device using tris(8-hydroxyquinolinol aluminum) in a light-emitting layer and a triphenyldiamine derivative to a hole-transporting layer. This stacked structure gives such advantages as an improvement in the injection efficiency of holes into the light-emitting layer; blocking of electrons injected from a cathode, which increase the efficiency of exciton production from charge recombination; and confinement of the excitons into the light-emitting layer. A double layered structure composed of a hole-injecting and transporting layer and an electron-transporting and light-emitting layer or a triple layered structure composed of a hole-injecting and transporting layer, a light-emitting layer and an electron-injecting and transporting layer is well known as an organic EL device. In order to increase the recombination efficiency of injected holes and electrons, various improvements in the device structure or fabrication process have been introduced to such multi-layered devices
However, in the organic EL device, the upper limit of a light-emitting probability exists because a probability of generating a singlet is restricted due to dependency on the spin statistics during the recombination of carriers. The upper limit is known to be about 25%. Further, in the organic EL device, light having an outgoing angle larger than the critical angle cannot be taken out because the light causes the total reflection due to the refractive index of the light-emitting member. When the refractive index of the light-emitting member is 1.6, only about 20% of the emitted light can be effectively utilized, and the upper limit of the total energy conversion efficiency including the singlet forming efficiency is forced to be about 5% (T. Tsutsui, xe2x80x9cCurrent State and Tend of Organic Electro-Luminescencexe2x80x9d, Monthly Display, vol.1, No.3, p.11, September, 1995). Such the low efficiency of the light-taking-out is a significant problem in the organic EL device in which the light-emitting is strictly restricted.
A means for elevating the light-taking-out efficiency of light emitting devices such as an inorganic EL device having similar structure to that of the above EL device has been examined. For example, the means includes a method for elevating an efficiency by providing an ability of collecting light to a substrate (JP-A-63(1988)-314795) and a method for forming a reflection surface on a side surface of a device (JP-A-1(1989)-220394). These methods are effective for a device having a larger light-emitting surface area, but the formation of the lens having the ability of collecting light and the reflection surface on the side surface are hardly performed in a device having a small pixel area such as a dot matrix display. Since the film thickness of the light-emitting layer of the organic EL device is several xcexcm or less, the formation of the reflection mirror having a tapered surface on the side surface of the device is difficult by using the current technique, and a high cost is required for performing the above formation. In another method in which a flattened layer having a refractive index between those of the substrate glass and the light-emitting layer is formed between the substrate glass and the light-emitting layer (JP-A-62(1987)-172691), the forward light-taking-out efficiency is improved but the total reflection cannot prevented Accordingly, even if the method is efficient the inorganic EL device having the larger refractive index, the method cannot exert the larger improving effect on the organic EL device which has the relatively lower refractive index.
Accordingly, the taking-out of the light in the conventional organic EL device is insufficient, and the development of the method for taking out the light in the organic EL device is inevitable. An organic EL device is described in JP-A-11(1999)-283751 including diffraction gratings for improving the light-taking-out efficiency. According to the method, the light-emitting efficiency is elevated, however, the light-taking-out efficiency is not sufficiently elevated.
In view of the foregoing, an object of the present invention is to provide an organic EL device having a higher efficiency and a method for fabricating the same.
The present invention provides, in a first aspect thereof, an organic EL device including a substrate having a main surface, at least one organic thin-film layer overlying the main surface, and a pair of electrodes sandwiching therebetween the organic thin-film layer, at least one of the electrodes being a metallic electrode, the organic thin film layer defining a recombination electroluminescence region which is apart from the metallic electrode by at least 100 nm in a direction normal to the main surface, the organic EL device having a periodic structure in a direction parallel to the main surface.
In accordance with the first aspect of the present invention, the emitted light is efficiently taken out by increasing the distance between the EL light-emitting region and the metal electrode for separating the light-emitting location from the metal electrode.
The present invention provides, in a second aspect thereof, a method of forming an organic EL device, wherein the organic EL device comprising a substrate having a main surface, a layered structure overlying the main surface and including at least one organic thin-film layer and a refractive layer having a refractive index higher than a refractive index of the organic thin-film layer, and a pair of electrodes sandwiching therebetween the organic thin-film layer, at least one of the electrodes being a transparent electrode, the organic EL device having a periodic structure in a direction parallel to the main surface, and wherein the method comprising the steps of applying a coating solution wherein a precursor including metallic compound is dispersed, and curing the coated film to form the refractive layer.
In accordance with the second aspect of the present invention, the refractive layer having both of the appropriate thickness and the transparency can be fabricated at a reduced cost by applying a coating solution in which a precursor containing a metal compound is dispersed followed by solidifying.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.